The present invention related in general to scanning equipment on a missile or other moving object, and in particular to a new and useful scanning prism which has reflecting surfaces that are inclined at a specific angle with respect to an axis of rotation for the prism.
For determination of the orientation parameters of a three line scanner aboard a missile or other moving object, there is a requirement for the generation of scanning traces which form a specific angle with respect to the flight path. Apparatus and methods are described in German patents Nos. DE 29 40 871 C 2, DE 30 43 577 C 2 and DE 32 19 032 C 2, which provide for the determination of the orientation parameters of a camera and a terrain model of the terrain being scanned, by means of opto-electronic scanners using three semiconductor sensor lines.
The German patent application No. P 35 17 671.7 expands on this method and utilizes it with opto-mechanical scanners. It is demonstrated in the above mentioned German patent No. 30 43 577, and investigations and publications based thereon (for instance Hoffmann, O.:, "Dynamische Photogrammetrie", Bildmessung und Luftbildwesen, No. 3, 1986), that the angular arrangements of the two outer sensor lines arranged in the image plane of a lens, together with the angular scanning traces produced by the lens, achieve a greater stability and accuracy of the model computation (orientation parameters and terrain points) than parallel arrangements for the sensor lines. In FIG. 1a of the present application the outer sensor lines or their traces in the terrain extend in a trapezoidal manner, and in FIG. 1b they are slightly arrow-shaped. In the extreme case of flight at low altitude it has proved favorable to change these arrow-shaped scanning traces of the outer sensors because of accuracy and stability, to such an extent, that the arrow tips point approximately toward the middle of the central sensor line or scanning trace and that the arrow angles are increased to approximately 30.degree. (FIG. 1c). It is not necessary that the scanning traces generated by the sensors extend precisely in a straight manner. They can, for instance, be slightly curved in a hyperbolic like fashion, as indicated by the dotted lines in FIG. 1c. In the above-mentioned German patent application No. P 35 17 671.7, the outer scanning traces are produced by rotating mirrors with mirror normal lines that are inclined with respect to the axis of rotation. This necessarily produces scanning traces similar in shape to a hyperbola.
Scanning with rotating mirrors, however, especially in case of large imaging angles of up to 180.degree., results in unwieldy large mirror surfaces, large apparatus dimensions and high moments of rotation or torque. In the following it is explained how these disadvantages can be avoided with a rotating prism of special design in accordance with the present invention.
It is known that in case of a two or four part scanning prism (see German patent application No. P 21 21 918.1, Scanning Prism), arranged in front of a lens 1 shown in FIG. 2a, two or four sweeps are produced in the course of one revolution of the prism 3 around its axis 7, depending on whether the prism is constructed in two or four parts. Herein, the bundle of rays on the optical axis 2 striking the prism at an angle .alpha. is deflected respectively in the direction of the rays 4 by a double amount 2.alpha.. The scanning traces 9 of the bundle of rays 4 as shown in FIG. 2b, produced in the plane of the object 8, extend perpendicularly to the axis of rotation 7 of the scanning prism (x-direction of the scanner), wherein the plane of the object 8 is located parallel to the axis of rotation 7 of the scanning prism and normally with respect to the optical axis 2 of the lens 1. In order to generate the scanning traces, which extend at a specific angle with respect to the x-axis of the scanner or the direction of flight (see FIGS. 1c, 3, 4), the reflecting surfaces 5 and 6 are changed in the present invention. In the known structures these reflecting surfaces 5 and 6 stand at right angles to each other and their line of intersection lies in the axis of rotation 7 of the scanning prism.
In the present invention, these reflecting surfaces are inclined at a specific angle .beta. with respect to the axis of rotation 7 of the scanning prism.